Updated July 5,
There's a coplanar
waveguide calculator on our calculator
Check out other
types of microwave transmission lines
here! Anyone have any CPW pictures they'd like to donate to this
page? Contact us!
Here's a clickable
index to our material on CPW:
of coplanar waveguide
Types of coplanar
and disadvantages of CPW
technology (separate page)
of coplanar waveguide
Coplanar waveguide was invented
by Cheng P. Wen, check out his picture in our Microwave
Hall of Fame! It was invented when Wen was at RCA's Sarnoff
Laboratories in New Jersey, way back in 1969, just two years after
the Summer of
Love... Dr. Wen now lives in Beijing, and continues to pass
on his considerable knowledge to microwave engineering students,
and he has corresponded with Microwaves101 on the topic of CPW (how's
that for name dropping?)
C.P. Wen explains that his original
name for coplanar waveguide was "planar strip line". A
co-worker, Lou Napoli suggested the name coplanar waveguide. Thus
with Lou's suggestion, CPW invented CPW!
waveguide (CPW) is formed from a conductor separated from a pair
of groundplanes, all on the same plane, atop a dielectric medium.
In the ideal case, the thickness of the dielectric is infinite;
in practice, it is thick enough so that EM fields die out before
they get out of the substrate.
A variant of coplanar waveguide
is formed when a ground plane is provided on the opposite side of
the dielectric, which is called finite ground-plane coplanar waveguide
(FGCPW), or more simply, grounded coplanar waveguide (GCPW).
The advantages of coplanar waveguide
are that active devices can be mounted on top of the circuit, like
on microstrip. More importantly, it can provide extremely high frequency
response (100 GHz or more) since connecting to CPW does not entail
any parasitic discontinuities in the ground plane. One disadvantage
is potentially lousy heat dissipation (this depends on the thickness
of the dielectric and whether it makes contact to a heat sink).
However, the main reason that CPW is not used is that there is a
general lack of understanding of how to employ it within the microwave
design community. I don't want to scare you away from CPW, but a
lot of CAD programs don't support it. This will change in the years
to come as more millimeter-wave work will demand the benefits of
We don't have equations for characteristic
impedance of CPW lines (yet). Neither does Microwave Engineering
by Pozar. If you want to get involved in CPW technology, a great
reference book is Coplanar Waveguide Circuits, Components and
Systems, by Simons. You'll find these books and more on our
recommended book page.
For a given line impedance, there
is an infinite number of solutions for the geometry of a CPW line.
You can make a fifty ohm line 10 microns wide, or 50 microns wide,
by adjusting the gap dimension. In practice, you'll tradeoff between
size of the circuitry and the line loss; skinny lines can become
If you are serious about designing
in CPW, consider purchasing Co-something
software. You can learn more about electronic design automation
software starting here.
CPW does not support a
TEM transmission mode. The filling
factor of ideal CPW (very thick substrate) is 50%. Therefore the
Keff is equal to:
about filling factor here.
Here's a rule of thumb for CPW: the effective dielectric constant
for "classic" CPW is very close to the average of the
dielectric constant of the substrate (because the filling factor
is 50%), and that of free space. If you are using GaAs, Er=12.9,
the effective dielectric constant would be (12.9+1)/2=6.95. One
way to think about this is that half of the electric field lines
are in free space, and half are in the dielectric.
and disadvantages of CPW
In terms of circuit isolation,
you can get great isolation using CPW, because there are always
RF grounds between traces. Many examples of high-isolation RF switches
have used grounded CPW to get 60 dB isolation or more.
The advantage of having a thick
substrate is realized when you fabricate CPW MMICs. The expense
of backside processing (thinning, via etch, backside plating) is
eliminated. Many companies that are currently developing GaN devices
are employing CPW so they can concentrate on device technology and
not have to figure out how to etch vias in silicon carbide or sapphire.
With GaN technology, wafer slices are on the order of 12 mils thick,
so for X-band devices, the height of the chip is well matched to
10 or 15 mil alumina.
For GaAs MMICs, wafer slices
start out at 25 mils. If a CPW chip is mounted face-up, a severe
height discontinuity can result. The way to get around this problem
is to use flip-chip technology, which
is an advantage or a disadvantage depending on who you talk to!
The ground inductance for shunt
elements is quite low for CPW, compared to microstrip applications.
This is because the RF ground is "right there", and you
don't have to drill a via hole to connect to it (vias add
As mentioned preciously, if you
want to make compact circuits using narrow transmission lines, you
must trade off RF loss. CPW circuits can be lossier than comparable
microstrip circuits, if you need a compact layout.
In terms of circuit size, CPW
is at a disadvantage versus a stripline of microstrip circuit, because
it's effective dielectric constant is lower (half of the fields
are in air).
Ground straps are always needed
to tie the two grounds together in CPW, or weird things can happen.
These are especially important around any discontinuity, such as
a tee junction.
Unintended spurious transmission
modes can also happen. In a parallel-plate mode, the substrate acts
like a dielectric-filled waveguide, and EM energy propagates along
unintended paths. Don't get us wrong, if you know how to avoid this
pitfall, CPW works great!
More to come!